Chromate-containing waste

Standard detoxification processes are used for chromate-containing wastes which are generated during surface treatment and finishing in metal working and metalplating facilities. These processes are also... 

Waste Waste containing lead-, zinc- and chromate-containing anticorrosive pigments that cannot be recycled must be taken to a special waste disposal site under proper control. 

Gemmell [250] has shown that chromate smelter waste containing calcium chromate from the soda ash and lime process, is phytotoxic at trace concentrations and that revegetation of affected areas is not effective unless the waste is initially covered with a 25 to 30 cm layer of subsoil [251J. 

Among the various sources of cadmium contamination are the plating operations and the disposal of cadmium-containing wastes. In these cases, the forms of cadmium also depend on the treatment of the waste prior to disposal. The most common forms include Cd ", cadmium-cyanide complexes, or Cd(OH)2 solid sludge. Hydroxide (Cd(OH)2) and carbonate (CdCOs) solids dominate at high pH, whereas Cd " and aqueous sulfate species are the dominant cadmium forms at pH <8. Under reducing conditions, when sulfide is present, the stable solid CdS(s> is formed. Cadmium also precipitates in the presence of phosphate, arsenate, chromate, and other anions, although solubility will vary with pH and other chemical factors [77]. The free Cd " appears to be the form readily taken up by plants, whereas CdCU is taken up more slowly, while Cd-humate is not adsorbed. 

Sodium titanate has been found to be very effective in removing Sr from defense waste typified by a 6m NaNO - 0.6m NaOH solution also containing the sodium salts of aluminate, nitrite, phosphate, carbonate, sulfate, and chromate in the range of O.lU to 0.007 N and Sr in the analytical concentration range of O.OU to O.U ppm ( ). Sodium titanate columns have provided a Sr decontamination factor of greater than io3 for 2500 column volumes of the waste at flow rates of 2 to 6 column volumes per hour. The material has also been shown to remove residual actinide contamination from the same and similar waste streams ( ). 

Waste chromium lyes containing chromic sulphate dissolved in sulphuric acid were formerly regenerated by the addition of lime in excess to neutralize the sulphuric acid and precipitate the chromium hydroxide. The precipitate of chromium hydroxide obtained was subject to oxidizing calcination in a rotary fumaee and converted to calcium chromate which in turn was converted to sodium bichromate as follows ... 

The EPA regulates both lead and hexavalent chromium in its hazardous waste regulation. Solid waste containing lead chromate or lead molybdate should be tested for toxicity prior to disposal via the TCLP test. 

As mentioned before, subsequent phosphate treatment does not affect the stable sulfide, and TCLP results show excellent stabihzation of Cr in any oxidation state. Alternatively, a small amount of reductant in the waste will convert chromate into lower oxidation states. Such methods, however, are not preferred, because the reductant may also affect the solubility of other hazardous compounds. The exception is technetium-containing radioactive waste, in which chromate is also a contaminant. As we shall see in Chapter 17, a reductant is essential for stabihzation of technetium, and that will also help in stabilization of chromium. 

The above reaction occurs rapidly at pH levels below 3. Because the acidic properties of ferrous sulfate are low at high dilutions, acid must be added for pH adjustment. The ferrous sulfate reducing process generates large volumes of sludge and thus its use is rare in large-scale treatment facilities. In addition, the use of ferrous sulfate to treat chromate wastes containing cyanide results in the formation of very stable ferrocyanide complexes, which prevent subsequent effective cyanide treatment. 

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